Systems and methods for full duplex coherent optics

ABSTRACT

A full duplex communication network includes an optical transmitter end having a first coherent optics transceiver, an optical receiver end having a second coherent optics transceiver, and an optical transport medium operably coupling the first coherent optics transceiver to the second coherent optics transceiver. The first coherent optics transceiver is configured to (i) transmit a downstream optical signal at a first wavelength, and (ii) simultaneously receive an upstream optical signal at a second wavelength. The second coherent optics transceiver is configured to (i) receive the downstream optical signal, and (ii) simultaneously transmit the upstream optical signal. The first wavelength has a first center frequency separated from a second center frequency of the second wavelength.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/274,152, filed Feb. 12, 2019, which application claims the benefit ofand priority to U.S. Provisional Patent Application Ser. No. 62/629,555,filed Feb. 12, 2018, and is a continuation in part of U.S. applicationSer. No. 16/198,396, which prior application claims the benefit of andpriority to U.S. Provisional Patent Application Ser. No. 62/589,121,filed Nov. 21, 2017, and to U.S. Provisional Patent Application Ser. No.62/636,249, filed Feb. 28, 2018. All of these applications areincorporated herein by reference in their entireties.

BACKGROUND

The field of the disclosure relates generally to communication networks,and more particularly, to bidirectional networks employing coherentoptics technologies.

Most network operators have very limited fiber available between theheadend (HE)/hub and the fiber node to use for data and video services,often only just 1-2 fiber strands. With end users demanding morebandwidth to the home, operators need a strategy on how to increasecapacity in the access network. One way is to add more fiber between theHE/hub and the fiber node, but retrenching is costly and time consuming,so return on investment (RoI) makes this option unattractive. A solutionthat re-uses the existing infrastructure is therefore considerablypreferable.

Coherent optics technology is becoming common in the subsea, long-haul,and metro networks, but has not yet been fully integrated into theaccess networks. However, it is desirable to utilize coherent opticstechnology in the access network because the distances from the HE/hubto the fiber node are much shorter using coherent optics technologies incomparison with conventional system technologies. One proposed techniquefor expanding the capacity of existing fiber infrastructures implementscoherent optics bidirectional transmission on a single fiber.Bidirectional transmission effectively doubles (or more) the amount oftransmission capability available to cable operators.

Bidirectional transmission is attractive to network operators that havelimited availability of leased or owned fibers, and who desireseparation of different services (residential, business, and cellularconnections) to end users/endpoints of the network. However, existingcoherent transceiver designs have been unable to fully leverage thecapabilities of bidirectional transmission. In particular, conventionalimplementations of single laser sources for both the transmitter and thelocal oscillator (LO) result in significant crosstalk that has preventedbidirectional transmission. Accordingly, it is desirable to developsystems and methods that successfully implement coherent opticstechnology in bidirectional transmission between the hub and the fibernode.

SUMMARY

In an embodiment, a communication network, includes an optical hubhaving a first coherent optics transceiver, a fiber node having a secondcoherent optics transceiver, an optical transport medium operablycoupling the first coherent optics transceiver to the second coherentoptics transceiver, a first optical circulator disposed at the opticalhub, and a second optical circulator disposed at the fiber node. Thefirst coherent optics transceiver is configured to (i) transmit adownstream optical signal at a first wavelength, and (ii) receive anupstream optical signal at the first wavelength. The second coherentoptics transceiver is configured to (i) receive the downstream opticalsignal from the first coherent optics transceiver at the firstwavelength, and (ii) transmit the upstream optical signal at the firstwavelength. The first and second optical circulators are configured toseparate the downstream optical signal from the upstream optical signal.

In an embodiment, a full duplex communication network includes anoptical transmitter end having a first coherent optics transceiver, anoptical receiver end having a second coherent optics transceiver, and anoptical transport medium operably coupling the first coherent opticstransceiver to the second coherent optics transceiver. The firstcoherent optics transceiver is configured to (i) transmit a downstreamoptical signal at a first wavelength, and (ii) simultaneously receive anupstream optical signal at a second wavelength. The second coherentoptics transceiver is configured to (i) receive the downstream opticalsignal, and (ii) simultaneously transmit the upstream optical signal.The first wavelength has a first center frequency separated from asecond center frequency of the second wavelength.

BRIEF DESCRIPTION

These and other features, aspects, and advantages of the presentdisclosure will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 is a schematic illustration of a coherent optics networkarchitecture.

FIG. 2 is a schematic illustration of a coherent optics networkarchitecture.

FIG. 3 is a schematic illustration of a coherent optics networkarchitecture.

FIG. 4 is a schematic illustration of a coherent optics networkarchitecture.

FIG. 5 is a schematic illustration of a coherent optics networkarchitecture.

FIG. 6 is graphical illustration of a comparative plot of bit error rateagainst received optical power.

FIG. 7 is graphical illustration of a comparative plot of bit error rateagainst received optical power.

FIG. 8 is graphical illustration of a superposition of the plotsdepicted in FIGS. 6 and 7.

FIG. 9 depicts a stimulated Brillouin scattering effect.

FIG. 10 is graphical illustration of a comparative plot of stimulatedBrillouin scattering threshold against fiber effective area.

FIG. 11 is graphical illustration of a comparative plot of stimulatedBrillouin scattering threshold against fiber effective area.

FIG. 12 is a schematic illustration of a coherent optics network testsystem.

FIG. 13 is graphical illustration of a comparative plot of measuredpower against input power utilizing the test system depicted in FIG. 12.

FIG. 14 is graphical illustration of an alternative comparative plot ofmeasured power against input power.

FIG. 15 is a zigzag reflection diagram of Rayleigh scattering.

FIG. 16 is a graphical illustration depicting a histogram of frequencyover optical return loss for a single mode fiber.

FIG. 17 is a graphical illustration depicting a comparative plot ofreflected power against input power.

FIG. 18 is a schematic illustration of a multipath interference sourcesystem.

FIG. 19 is graphical illustration depicting a comparative plot ofoptical signal-to-noise ratio penalty as a function of multipathinterference.

FIG. 20 depicts alternative fiber connector structures.

FIG. 21 is a schematic illustration of a coherent optics networkarchitecture.

FIG. 22 is a schematic illustration of a coherent optics networkarchitecture.

FIG. 23 is a schematic illustration of a coherent optics networkarchitecture.

FIG. 24 is a schematic illustration of a coherent optics networkarchitecture.

FIG. 25 is a schematic illustration of a coherent optics networkarchitecture.

FIG. 26 is a schematic illustration of a coherent optics networkarchitecture.

FIG. 27 is a schematic illustration of a coherent optics networkarchitecture.

FIG. 28 is a graphical illustration of a comparative optical spectrumplot for a single channel.

FIG. 29 is a graphical illustration of a comparative optical spectrumplot for a wavelength division multiplexing channel.

FIG. 30 is a graphical illustration of a comparative optical spectrumplot for a C-Band channel.

FIG. 31 is a graphical illustration depicting a comparative plot ofreflected power against input power.

FIG. 32 is a graphical illustration depicting a comparative plot ofreflected power against input power.

FIG. 33 is a graphical illustration depicting a comparative plot ofreflected power against input power.

FIG. 34 is a graphical illustration depicting a comparative plot ofreflected power against input power.

FIG. 35 is a graphical illustration depicting a comparative plot ofreflected power against input power.

FIG. 36 is a graphical illustration depicting a comparative plot ofreflected power against input power.

FIG. 37 is a schematic illustration of a coherent optics networkarchitecture.

FIG. 38 is a schematic illustration of a coherent optics networkarchitecture.

FIG. 39 is a schematic illustration of an exemplary optical networkunit.

FIG. 40 is a schematic illustration of an exemplary optical networkunit.

FIG. 41 is a schematic illustration of an exemplary optical networkunit.

FIG. 42 is a schematic illustration of an exemplary optical conversionarchitecture.

FIG. 43 is a graphical illustration depicting relative signaldistributions for the architecture depicted in FIG. 37.

FIG. 44 is a graphical illustration depicting relative signaldistributions for the architecture depicted in FIG. 38.

Unless otherwise indicated, the drawings provided herein are meant toillustrate features of embodiments of this disclosure. These featuresare believed to be applicable in a wide variety of systems including oneor more embodiments of this disclosure. As such, the drawings are notmeant to include all conventional features known by those of ordinaryskill in the art to be required for the practice of the embodimentsdisclosed herein.

DETAILED DESCRIPTION

In the following specification and claims, reference will be made to anumber of terms, which shall be defined to have the following meanings.

The singular forms “a,” “an,” and “the” include plural references unlessthe context clearly dictates otherwise.

“Optional” or “optionally” means that the subsequently described eventor circumstance may or may not occur, and that the description includesinstances where the event occurs and instances where it does not.

As used herein, unless specified to the contrary, “modem terminationsystem,” or “MTS′” may refer to one or more of a cable modem terminationsystem (CMTS), an optical network terminal (ONT), an optical lineterminal (OLT), a network termination unit, a satellite terminationunit, and/or other termination devices and systems. Similarly, “modem”may refer to one or more of a cable modem (CM), an optical network unit(ONU), a digital subscriber line (DSL) unit/modem, a satellite modem,etc.

As used herein, the term “database” may refer to either a body of data,a relational database management system (RDBMS), or to both, and mayinclude a collection of data including hierarchical databases,relational databases, flat file databases, object-relational databases,object oriented databases, and/or another structured collection ofrecords or data that is stored in a computer system.

Furthermore, as used herein, the term “real-time” refers to at least oneof the time of occurrence of the associated events, the time ofmeasurement and collection of predetermined data, the time for acomputing device (e.g., a processor) to process the data, and the timeof a system response to the events and the environment. In theembodiments described herein, these activities and events occursubstantially instantaneously.

Approximating language, as used herein throughout the specification andclaims, may be applied to modify any quantitative representation thatcould permissibly vary without resulting in a change in the basicfunction to which it is related. Accordingly, a value modified by a termor terms, such as “about,” “approximately,” and “substantially,” are notto be limited to the precise value specified. In at least someinstances, the approximating language may correspond to the precision ofan instrument for measuring the value. Here and throughout thespecification and claims, range limitations may be combined and/orinterchanged; such ranges are identified and include all the sub-rangescontained therein unless context or language indicates otherwise.

The embodiments described herein provide innovative systems and methodsfor full-duplex coherent optics, that is, bidirectional (BiDi) coherentoptics networks. The present techniques may further advantageouslyimplement passive optical circulators and a variety of versatilearchitectural configurations to separate the upstream and downstreamsignal flows of the BiDi network. According to these embodiments,spectral efficiency is significantly improved in both the downstream andupstream directions. As described further herein, both the downstreamand upstream transmissions may utilize the same wavelength andsimultaneous transmission over the same fiber, thereby doubling thespectral efficiency of existing coherent transmission systems ornetworks.

FIG. 1 is a schematic illustration of a coherent optics networkarchitecture 100. In the example depicted in FIG. 1, architecture 100represents an aggregation use case for a distributed coherent opticsnetwork. Architecture 100 includes a hub 102, a node 104, and atransport medium 106 (e.g., an optical fiber) communicatively coupledtherebetween. In an exemplary embodiment, transport medium 106 is asingle strand fiber for a coherent optic link. Architecture 100 furtherincludes a hub coherent transceiver 108 and a hub optical circulator 110(e.g., a three-port optical circulator in the illustrated example) athub 102. Similarly, architecture 100 includes a node coherenttransceiver 112 and a node optical circulator 114.

In an exemplary embodiment, coherent transceivers 108, 112 include asingle laser source, a transmitting portion, and a receiving portion,respectively (not separately numbered). In operation, architecture 100is configured for bidirectional transmission of a wavelength λ in boththe downstream (DS) and upstream (US) directions. In particular,architecture 100 represents transmission over a single channel (e.g.,100G, 200G, etc.), where both coherent transceivers 108, 112 use theirrespective single laser sources for both transmitter LO and receiver LO.That is, the wavelength λ is the same for both the downstream andupstream transmission.

Exemplary architectures of coherent hub-to-node networks and systems aredescribed in greater detail in co-pending U.S. patent application Ser.No. 15/283,632, filed Oct. 3, 2016, and co-pending U.S. patentapplication Ser. No. 15/590,464, filed May 9, 2017, the disclosures ofboth which are incorporated by reference herein. Additionally, theperson of ordinary skill in the art will understand that architecture100 is simplified for ease of explanation, does not necessarilyillustrate all components that may be implemented within a hub and/orfiber node.

FIG. 2 is a schematic illustration of a coherent optics networkarchitecture 200. Architecture 200 is similar to architecture 100, FIG.1, and also represents an example of an aggregation use case.Accordingly, architecture 200 includes a hub 202, a fiber node 204, atransport medium/fiber 206, a hub coherent transceiver 208, a huboptical circulator 210, a node coherent transceiver 212, and a nodeoptical circulator 214. In the example depicted in FIG. 2, hub coherenttransceiver 208 includes a hub transmitter 216 and a separate hubreceiver 218. Similarly, node coherent transceiver 212 includes a nodereceiver 220 and a node transmitter 222.

In an exemplary embodiment, architecture 200 is configured to implementtransmission over a dense wavelength division multiplexing (DWDM)channel, and further includes a first optical splitter 228 at hub 202and a second optical splitter 230 at node 204. In an embodiment,architecture 200 further includes a first optical multiplexer 224 at hub202 and a second optical multiplexer 226 at node 204. In this example,architecture 200 is configured to transmit multiple wavelengths λ₁, λ₂,. . . λ_(N) in both directions. In the example depicted in FIG. 2, ademultiplexer is optionally the necessary at coherent receivers 218, 220where the respective LO serves for signal selectivity. This embodimentmay, for example, be particularly advantageous in the case of a limitednumber of DWDM channels.

FIG. 3 is a schematic illustration of a coherent optics networkarchitecture 300. Architecture 300 is similar to architecture 200, FIG.2, and also represents an example of an aggregation use case and isconfigured to implement DWDM transmission. Accordingly, architecture 300includes a hub 302, a fiber node 304, a transport medium/fiber 306, ahub coherent transceiver 308, a hub optical circulator 310, a nodecoherent transceiver 312, a node optical circulator 314, a hubtransmitter 316, a hub receiver 318, a node receiver 320, a nodetransmitter 322, a first optical multiplexer 324, and a second opticalmultiplexer 326. Architecture 300 differs though, from architecture 200in that architecture 300 further includes a first optical demultiplexer328 at hub 302 and a second optical demultiplexer 330 at node 304. Thatis, architecture 300 effectively replaces first optical splitter 228 andsecond optical splitter 230 with first optical demultiplexer 328 andsecond optical demultiplexer 330, respectively.

FIG. 4 is a schematic illustration of a coherent optics networkarchitecture 400. Architecture 400 is similar to architecture 300, FIG.3, and also represents an example of an aggregation use case for DWDMchannels. Accordingly, architecture 400 includes a hub 402, a fiber node404, a transport medium/fiber 406, a hub coherent transceiver 408, a huboptical circulator 410, a node coherent transceiver 412, a node opticalcirculator 414, a hub transmitter 416, a hub receiver 418, a nodereceiver 420, a node transmitter 422, a first optical multiplexer 424, asecond optical multiplexer 426, a first optical demultiplexer 428, and asecond optical demultiplexer 430. Architecture 400 differs though, fromarchitecture 300 in that architecture 400 further includes a boostamplifier 432 between first optical multiplexer 424 and hub opticalcirculator 410, and a pre-amplifier 434 between hub optical circulator410 and first optical demultiplexer 428. In an exemplary embodiment,boost amplifier 432 and pre-amplifier 434 are erbium-doped fiberamplifiers (EDFAs) functioning as optical repeater devices that boostthe intensity of optical signals being carried through the fiber opticcommunications system of architecture 400.

FIG. 5 is a schematic illustration of a coherent optics networkarchitecture 500. Architecture 500 is similar to architecture 400, FIG.4, and also represents an example of an aggregation use case for DWDMchannels. Accordingly, architecture 500 includes a hub 502, a fiber node504, a transport medium/fiber 506, a hub coherent transceiver 508, a huboptical circulator 510, a node coherent transceiver 512, a node opticalcirculator 514, a hub transmitter 516, a hub receiver 518, a nodereceiver 520, a node transmitter 522, a first optical multiplexer 524, asecond optical multiplexer 526, a first optical demultiplexer 528, asecond optical demultiplexer 530, a hub boost amplifier 532, and a hubpre-amplifier 534. Architecture 500 differs though, from architecture400 in that architecture 500 further includes a node pre-amplifier 538between node optical circulator 514 and second optical demultiplexer530, and a node boost amplifier 538 between second optical multiplexer526 and node optical circulator 514. In an exemplary embodiment, nodepre-amplifier 536 and node boost amplifier 432 are also EDFAs.

The several architectures described herein were subject to proof ofconcept, which produced significant preliminary experimental results. Inexemplary experimentation, forward error correction (FEC) encoding wasemployed (e.g., staircase FEC). Some of the FEC results reflect the useof hard decision (HD) FEC (HD-FEC) for case of 100G with 7% overhead,staircase FEC. In one particular embodiment, approximately a 1 dB powerpenalty was had for a 7% staircase FEC at 4.5e−3 for both directions insingle channel 100G testing (single channel case).

A difference may then be seen between the upstream and downstreamtransmissions due to the sensitivity differences of the respectivecoherent receivers. However, after correction by HD-FEC techniques, noerror was found over a 80-km transmission. Nevertheless, the differentoutput powers from the respective coherent transmitters exhibits anoticeable impact on the link receiver sensitivity. Accordingly, for aparticular transmission link, it is further desirable to utilize thepresent techniques to optimize output power to minimize the powerpenalty, as described further below. The experimental results describedherein also consider various parameters of the respective opticalcirculators as featured below in Table 1.

TABLE 1 Wavelength Range nm 1525-1610 Insertion Loss Port 1 → 2 dB 0.73Port 2 → 3 0.66 PDL Port 1 → 2 dB 0.05 Port 2 → 3 0.04 Return Loss Port1 dB 60 Port 2 60 Port 3 60 Isolation at 1570 nm Port 2 → 1 52 Port 3 →2 57 Directivity Port 1 → 3 60 Port 3 → 1 55 PMD ps <0.05

FIG. 6 is graphical illustration of a comparative plot 600 of bit errorrate (BER) against received optical power. In an exemplary embodiment,plot 600 represents the received optical power over an 80-km single modefiber (SMF), such as an SMF-28, and for a transmitter output power of −8dBm (without EDFAs, in this example) and a 4.5e−3 staircase FECthreshold. Plot 600 includes a first sub-plot 602 representing adownstream transmission in a bidirectional use case, a second sub-plot604 representing a downstream transmission in a single direction usecase, a third sub-plot 606 representing an upstream transmission in thebidirectional use case, and a fourth sub-plot 608 representing anupstream transmission in the single direction use case. As can be seenfrom the example depicted in FIG. 6, the received optical power isconsistently greater in the bidirectional case, in both the downstreamand upstream directions.

FIG. 7 is graphical illustration of a comparative plot 700 of BERagainst received optical power. Plot 700 is similar to plot 600, FIG. 6,in that plot 700 represents the received optical power over an 80-kmSMF-28, a 4.5e−3 staircase FEC threshold, and without EDFA. Plot 700differs from plot 600 though, in that plot 700 represents theexperimental results for a transmitter output power of 0 dBm. Plot 700includes a first sub-plot 702 representing a downstream transmission inthe bidirectional use case, a second sub-plot 704 representing adownstream transmission in the single direction use case, a thirdsub-plot 706 representing an upstream transmission in the bidirectionaluse case, and a fourth sub-plot 708 representing an upstreamtransmission in the single direction use case. As can be seen from theexample depicted in FIG. 7, the received optical power is againconsistently greater in the bidirectional case, in both the downstreamand upstream directions.

FIG. 8 is graphical illustration of a superposition 800 of plot 600,FIG. 6, and plot 700, FIG. 7. Superposition 800 illustrates how thereceived optical power generally tracks with the results of differenttransmitter output powers, and is generally higher as the transmitteroutput power increases, except in the bidirectional downstreamtransmission.

From the preliminary results of the embodiments described immediatelyabove, additional analysis of testing results were obtained severalimplementations of full duplex coherent optics architectures andsystems. Conventional full duplex coherent optics systems are subject tosignificant impairments, including: (i) Stimulated Brillouin Scattering(SBS); (ii) Rayleigh scattering (continuous reflection); (iii)Multiple-Path/Multipath Interference (MPI), for coherent or incoherentinterference, and including double-Rayleigh scattering, localreflections (mechanical splices, fusion, jumper cables, etc.), and/oroptical amplifiers; and (iv) Fresnel reflection (discrete reflections),including jumper cables, optical distribution panels, fusion, mechanicalsplices, etc.

FIG. 9 depicts a SBS effect 900. SBS effect 900 occurs, for example,where variations in the electric field of an incident beam of light(e.g., from a laser) traveling through a transport medium (e.g., anoptical fiber), induce acoustic vibrations (i.e., an acoustic wave) inthe fiber by electrostriction or radiation pressure. Brillouinscattering (e.g., scattered light) thus occurs, in the directionopposite the incident light beam as a result of the acoustic wavevibrations, as illustrated in FIG. 9. More particularly, SBS effect 900occurs from the interaction between the light and acoustic waves. Thepropagating light beam in the fiber generates a propagating acousticwave that creates a periodic variation of the fiber refractive index.The back-scattered wave, also referred to as a Stokes wave, isdownshifted by approximately 11 GHz with respect to the incident lightwave frequency. When increasing the launched power of the optical beam,the reflected power will increase linearly as a result of the Rayleighback-scattering effect in the fiber. Above a given threshold, thereflected power will then increase exponentially due to SBS effect 900.

FIG. 10 is graphical illustration of a comparative plot 1000 of SBSthreshold against fiber effective area. Comparative plot 1000 includes afirst sub-plot 1002 representing a case of a 200 MHz linewidth, and asecond sub-plot 1004 representing a case of a 20 MHz linewidth. As canbe seen from the example depicted in FIG. 10, the SBS threshold isconsiderably greater as the linewidth increases.

In an exemplary embodiment, SBS threshold (Power_th) for an unmodulatedcontinuous wave (CW) may be represented as:

${{Power\_ th}( {{B \cdot g_{b}},\alpha_{dB},A_{eff},{Length},{\Delta\; v_{S}},{\Delta\; v_{B}}} )}:={\frac{21 \cdot B \cdot A_{eff}}{g_{b} \cdot \frac{1 - e^{{- \alpha_{dB}} \cdot \frac{\ln{(10)}}{10} \cdot {Length}}}{\alpha_{dB} \cdot \frac{\ln(10)}{10}}} \cdot ( {1 + \frac{\Delta\; v_{s}}{\Delta\; v_{B}}} )}$

Where B is a number between 1 and 2 of a polarization state, A_(eff) isthe effective area of fiber, g_(b) is an SBS gain coefficient, Length isthe fiber distance, α_(dB) is a fiber attenuation coefficient, Δν_(S) isa linewidth of signal source, and Δν_(B) is an SBS interactionbandwidth. In the example depicted in FIG. 10, the SBS threshold for theunmodulated CW was Power_th(1, 4*10⁻¹¹, 0.0002, A_(eff), 50*10³, 20*10⁶,20*10⁶).

FIG. 11 is graphical illustration of a comparative plot 1100 of SBSthreshold against fiber effective area. Comparative plot 1100 is similarto comparative plot 1000, FIG. 10, except that comparative plot 1100depicts a comparison of different baud levels, as opposed to differentlinewidths. More specifically, comparative plot 1100 includes a firstsub-plot 1102 representing a 32 GBaud case, and a second sub-plot 1004representing a 28 GBaud case. As can be seen from comparative plot 1100,the SBS threshold is greater as the baud increases. In the exampledepicted in FIG. 11, the experimental results were gathered usingPM-QPSK signals over a 50-km (19.54-km effective length) transmission,the SBS threshold for the PM-QPSK signals was Power_th(1, 4*10−11,0.0002, A_(eff), 50*10³, 28*10⁹, 20*10⁶).

FIG. 12 is a schematic illustration of a coherent optics network testsystem 1200. Test system 1200 was used to obtain, from an input powersource 1202, measured power results 1204, as described further belowwith respect to FIGS. 13 and 14, for a CW source (e.g., comparative plot1000, FIG. 10) and a QPSK source (e.g., comparative plot 1100, FIG. 11),respectively.

FIG. 13 is graphical illustration of a comparative plot 1300 of measuredpower against input power utilizing test system 1200, FIG. 12, in asimulation. In an exemplary embodiment, the input power isrepresentative of a CW source, and the measured power is for a 20 MHzlinewidth over a 20-km SMF-28. In this example, the measured power ofcomparative plot 1300 includes a first sub-plot 1302 representing thetransmitted signal power, and a second sub-plot 1304 representing theStokes power. As can be seen from the example depicted in FIG. 13, themeasured transmitted signal power 1302 is significantly greater than theStokes power 1304 until the input power reaches approximately 7 dBm,above which the measured Stokes power 1304 exceeds the measuredtransmitted signal power.

FIG. 14 is graphical illustration of an alternative comparative plot1400 of measured power against input power. Comparative plot 1400 issimilar to comparative plot 1300, FIG. 13, except that comparative plot1400 demonstrates the result of the simulation for a QPSK (32 GBaud)source utilizing test system 1200, FIG. 12, in in alternativesimulation. In this example, the measured power of comparative plot 1400includes a first sub-plot 1402 representing the transmitted signalpower, and a second sub-plot 1404 representing the Stokes power. As canbe seen from the example depicted in FIG. 14, the measured transmittedsignal power 1402 is consistently greater than the Stokes power 1404across the entire range of input power levels.

Accordingly, in the case of SBS in coherent optic systems, because ofthe effect of phase-modulated signals on the reduction of opticalcarrier power, the effective linewidth is proportional to the signalbaud rate. Accordingly, the SBS threshold power will significantlyincrease in a similar manner. However, the SBS was found to benegligible for a launch power less than 7 dBm/channel in the coherentoptical systems described above with respect to FIG. 13 (CW source).

Simulations in consideration of Rayleigh scattering impairments aredescribed further below with respect to FIGS. 15-17.

FIG. 15 is a zigzag reflection diagram 1500 of Rayleigh scattering.Reflection diagram 1500 demonstrates the significant of time anddistance with respect to the scattering effect. FIG. 16 is a graphicalillustration depicting a histogram 1600 of frequency against opticalreturn loss (ORL) for an SMF-28. The example depicted in FIG. 16illustrates a case of a non-dispersion-shifted fiber (NDSF) SMF-28.

FIG. 17 is a graphical illustration depicting a comparative plot 1700 ofreflected power against input power. Comparative plot 1700 depictssimulated results at a 1548.52 nm wavelength, and includes sub-plots1702 representative of a 35 dB reflection power measured over differenttransmission distances of 26-km, 52-km, and 78-km, respectively. As canbe seen from the example depicted in FIG. 17, the reflected power tracksfairly linearly with the input power, and is substantially agnostic ofthe various changes to the transmission distances.

FIG. 18 is a schematic illustration of an MPI source system 1800. In theembodiment depicted in FIG. 18, system 1800 is utilized to produce MPIinterference 1802 over eight separate paths (i.e., 5-km, 10-km, 15-km,20-km, 25-km, 30-km, 35-km, and 40-km delays, in this example). In anexemplary embodiment, system 1800 implements a plurality of variableoptical attenuators (VOAs).

FIG. 19 is graphical illustration depicting a comparative plot 1900 ofoptical signal-to-noise ratio (OSNR) penalty as a function of MPI (e.g.,FIG. 18). In this example, comparative plot 1900 includes a firstsub-plot 1902 representative of a 3.8e−3 BER, and a second sub-plot 1904representative of a 1.9e−3 BER. As can be seen from the example depictedin FIG. 19, the OSNR penalty increases exponentially as a function ofMPI, and that this effect is significantly greater as the BER increases.Nevertheless, sub-plot 1902 demonstrates that, at 3.8e−3 BER,approximately 1 dB of OSNR penalty can be observed for −16 dBc of MPI,which indicates a significantly high tolerance to MPI. In the exampledepicted in FIG. 19, the results were obtained in consideration of thepost-FEC error count against the pre-FEC BER, which included asubstantial range having zero post-FEC errors.

FIG. 20 depicts alternative fiber connector structures 2000, 2002, 2004,2006 having different respective ferrule end finishes to reducereflectance/loss. More specifically, structure 2000 represents a flatfiber optic end finish (e.g., less than −30 dB back reflection),structure 2002 represents a physical contact (PC) fiber optic end finish(e.g., less than −35 dB back reflection), structure 2004 represents anultra physical contact (UPC) fiber optic end finish (e.g., less than −55dB back reflection), and structure 2006 represents an angled physicalcontact (APC) optical end finish (e.g., less than −65 dB backreflection). APC structure 2006 is often found in existing hybrid fibercoaxial (HFC) networks, whereas UPC structure 2004 is often consideredfor networks having a relatively small number of digital links.

The following embodiments describe additional systems and methods forexperimental analysis and lab testing for further proof of concept fromthe experimental results obtained thereby. More particularly, theembodiments depicted in FIGS. 21-27 generally correspond with theseveral hub-to-fiber node architectures depicted in FIGS. 1-5, describedabove, but are addressed more generally to the full duplex paradigm ofbidirectionality, which may be more significantly agnostic of whichdirection is considered “downstream” versus “upstream.”

FIG. 21 is a schematic illustration of a coherent optics networkarchitecture 2100. In an exemplary embodiment, architecture 2100includes a first coherent transceiver 2102 in operable communicationwith a second coherent transceiver 2104 over an SMF 2106. First coherenttransceiver 2102 includes a first transmitter portion 2108 and secondcoherent transceiver 2104 includes a second transmitter portion 2110.Similarly, second coherent transceiver 2104 includes a first receiverportion 2112, and first coherent transceiver 2102 includes a secondreceiver portion 2114. First transmitter portion 2110 and secondreceiver portion 2114 communicate with fiber 2106 through a firstoptical circulator 2116, and first receiver portion 2112 and secondtransmitter portion 2110 communicate with fiber 2106 through a secondoptical circulator 2118.

In exemplary operation of architecture 2100, first and secondtransmitter portions 2108, 2110 both transmit at X-dBm of transmitpower, and fiber 2106 is subject to Y-dB loss. Accordingly, architecture2100 should function such that values for X−Y≥−30 dBm (e.g., thereceiver sensitivity), and that values for [(X−Y)−(X−35)]≥15.4 dB (e.g.,the OSNR, however, larger OSNR values are contemplated due to only 0.1nm noise power included in this example). Further to this example, theloss Y should be such that Y(loss)≤19.6 dB.

FIG. 22 is a schematic illustration of a coherent optics networkarchitecture 2200. Architecture 2200 is similar to architecture 2100,FIG. 21, and similarly includes a first coherent transceiver 2202, asecond coherent transceiver 2204, an SMF 2206, a first transmitterportion 2208, a second transmitter portion 2210, a first receiverportion 2212, a second receiver portion 2214, a first optical circulator2216, and a second optical circulator 2218. Architecture 2200 differsfrom architecture 2100 though, in that architecture 2200 furtherincludes a first optical multiplexer 2220 at first transceiver 2202 anda second optical multiplexer 2222 at second transceiver 2204, and also afirst optical demultiplexer 2224 at second transceiver 2204 and a secondoptical demultiplexer 2226 at first transceiver 2202. Thisdual-multiplexer/demultiplexer configuration operates similarly toarchitecture 300, FIG. 3.

In the example depicted in FIG. 22, first and second transmitterportions 2208, 2210 operate at 3 (e.g., X) dBm of transmit power, andthe sensitivity of first receiver portion 2212 is −30 dBm receivedpower. According to the calculations described above, the total loss Ywill be (X−receiver sensitivity), which is [3−(−30)], or 33 dB. Furtherin this example, loss at each optical circulator 2216, 2218 is 2 dB, andloss at the multiplexers/demultiplexers is 5 dB each. Accordingly, thefiber loss may then be calculated as [33−(5+2)*2], or 19 dB. Thereflected power before second optical circulator 2218 is [−33−(5+2)], or−39 dB, and the reflected power at receiver portion 2212 will be[−39−(2+5)], or −46 dBm. From these values, the OSNR is found from[−30−(−46)], or 16 dB.

FIG. 23 is a schematic illustration of a coherent optics networkarchitecture 2300. Architecture 2300 is also similar to architecture2100, FIG. 21, and similarly includes a first coherent transceiver 2302,a second coherent transceiver 2304, an SMF 2306, a first transmitterportion 2308, a second transmitter portion 2310, a first receiverportion 2312, a second receiver portion 2314, a first optical circulator2316, and a second optical circulator 2318, with both first and secondtransmitter portions 2308, 2310 operating at 3 dBm of transmit power. Inthe example depicted in FIG. 23 fiber 2306 includes a 51.8-km portionand a 26.3-km portion, and exhibits a 17.8 dB fiber loss (i.e.,approximately 18 dB total common link loss). Accordingly, in thisembodiment, first receiver portion 2312 has a BER of 1.3e−6, and secondreceiver portion 2314 has a BER of 5.14e−7.

FIG. 24 is a schematic illustration of a coherent optics networkarchitecture 2400. Architecture 2400 is also similar to architecture2300, FIG. 23, and similarly includes a first coherent transceiver 2402,a second coherent transceiver 2404, an SMF 2406 (17.8 dB fiber loss, inthis example also), a first transmitter portion 2408, a secondtransmitter portion 2410, a first receiver portion 2412, a secondreceiver portion 2414, a first optical circulator 2416, and a secondoptical circulator 2418, with both first and second transmitter portions2408, 2410 operating at 3 dBm of transmit power and at approximately 18dB total common link loss. Architecture 2400 differs from architecture2300 though, in that architecture 2400 further includes a firstattenuator 2420 between first transmitter portion 2408 and first opticalcirculator 2416, and a second attenuator 2422 between second opticalcirculator 2418 and first receiver portion 2412, with each attenuator2420, 2422 having 3 dB of attenuation. Accordingly, in this embodiment,first receiver portion 2412 has a BER of 2.48e−6, and second receiverportion 2414 has a BER of 1.19e−6.

FIG. 25 is a schematic illustration of a coherent optics networkarchitecture 2500. Architecture 2500 is also similar to architecture2300, FIG. 23, and similarly includes a first coherent transceiver 2502,a second coherent transceiver 2504, an SMF 2506 (17.8 dB fiber loss, inthis example also), a first transmitter portion 2508, a secondtransmitter portion 2510, a first receiver portion 2512, a secondreceiver portion 2514, a first optical circulator 2516, and a secondoptical circulator 2518. Different from architecture 2400 though, in theexample depicted in FIG. 25, first and second transmitter portions 2508,2510 are configured to operate at various transmit power levels.

More particularly, architecture 2500 operates according to a first case,where the transmit power of first transmitting portion 2508 varies from−5 dBm through −10 dBm, while the transmit power of second transmittingportion 2510 remains constant at −5 dBm. Accordingly, the BER values atfirst receiver portion 2512 correspondingly change, as reflected intable 2520. Similarly, architecture 2500 operates according to a secondcase, where the transmit power of second transmitting portion 2510varies from 0 dBm through −5 dBm, while the transmit power of firsttransmitting portion 2508 remains constant at −5 dBm. Accordingly, theBER values at second receiver portion 2514 correspondingly change, asreflected in table 2522.

FIG. 26 is a schematic illustration of a coherent optics networkarchitecture 2600. Architecture 2600 is similar to architecture 2400,FIG. 24, and similarly includes a first coherent transceiver 2602, asecond coherent transceiver 2604, an SMF 2606 (17.8 dB fiber loss, inthis example also), a first transmitter portion 2608, a secondtransmitter portion 2610, a first receiver portion 2612, a secondreceiver portion 2614, a first optical circulator 2616, a second opticalcirculator 2618, a first attenuator 2620 (3 dB), and a second attenuator2622 (3 dB), with both first and second transmitter portions 2608, 2610operating at 3 dBm of transmit power. Architecture 2600 differs fromarchitecture 2400 though, in that architecture 2600 further includes,between first and second optical circulators 2416, 2418, third andfourth attenuators 2624, 2422, each having 5 dB of attenuation, therebyresulting in approximately 28 dB of total common link loss (i.e., 18dB+(5 dB)*2), and a BER of 2.14e−4 at first receiver portion 2612 and aBER of 1.549e−4 at second receiver portion 2614.

FIG. 27 is a schematic illustration of a coherent optics networkarchitecture 2700. Architecture 2700 is also similar to architecture2400, FIG. 24, and similarly includes a first coherent transceiver 2702,a second coherent transceiver 2704, an SMF 2706 (50-km single fiber, inthis example), a first transmitter portion 2708, a second transmitterportion 2710, a first receiver portion 2712, a second receiver portion2714, a first optical circulator 2716, a second optical circulator 2718,and a first attenuator 2720 between first transmitter portion 2708 andfirst optical circulator 2716. Architecture 2700 differs fromarchitecture 2400 though, in that architecture 2700 further includes asecond attenuator 2722 between second optical circulator 2718 and secondtransmitter portion 2710. Accordingly, in this example, each attenuator2720, 2722 results in −20 dBm transmit power seen at the respectiveoptical circulator.

The architectural embodiments described above are depicted with respectto single channel operation, for ease of explanation. In an exemplaryspectrum plot of single channel operation is described further belowwith respect to FIG. 28. The person of ordinary skill in the art,however, will understand how the present systems and methods may beapplied with respect to WDM operations as well. Some exemplary resultsof WDM operation, in accordance with the present embodiments, aredescribed further below with respect to FIGS. 29-35.

FIG. 28 is a graphical illustration depicting a comparative opticalspectrum plot 2800 for a single channel. Comparative optical spectrumplot 2800 is representative of power over wavelength for a singlechannel operation (at 0.1-nm resolution, in this example), and includesa first sub-plot 2802 illustrating the downstream optical spectrum ofthe single channel, and a second sub-plot 2804 illustrating the upstreamoptical spectrum of the single channel. As can be seen from the exampledepicted in FIG. 28, upstream optical spectrum 2804 tracks fairlyclosely with downstream optical spectrum 2802, with downstream opticalspectrum 2802 being slightly greater about a central wavelength (1548.52nm, in the illustrated example).

FIG. 29 is a graphical illustration depicting a comparative opticalspectrum plot 2900 for a WDM channel. Comparative optical spectrum plot2900 is representative of power over wavelength for a two-wavelength WDMchannel operation (e.g., again at 0.1-nm resolution), and includes afirst sub-plot 2902 illustrating the downstream optical spectrum of theWDM channel, and a second sub-plot 2904 illustrating the upstreamoptical spectrum of the WDM channel. As can be seen from the exampledepicted in FIG. 29, upstream optical spectrum 2904 tracks fairlyclosely with downstream optical spectrum 2902, however, in this case,downstream optical spectrum 2902 has slightly lower power about thecentral peak wavelengths of the WDM channel (1547.57 nm and 1548.52 nm,in the illustrated example).

FIG. 30 is a graphical illustration depicting a comparative opticalspectrum plot 3000 for a C-Band channel. Comparative optical spectrumplot 3000 is representative of power over wavelength for a C-Bandchannel operation (e.g., again at 0.1-nm resolution), and includes afirst sub-plot 3002 illustrating the downstream optical spectrum of theC-Band channel, and a second sub-plot 3004 illustrating the upstreamoptical spectrum of the C-Band channel. As can be seen from the exampledepicted in FIG. 30, upstream optical spectrum 3004 tracks fairlyclosely with downstream optical spectrum 3002 about the central peakwavelengths of the C-Band channel (e.g., 1547.57 nm and 1548.52 nm, inthe illustrated example), but upstream optical spectrum 3004 exhibits aconsiderably higher noise floor outside of the peak wavelengths. Thebackscattering noise power is described further below with respect toFIGS. 31-35.

FIG. 31 is a graphical illustration depicting a comparative plot 3100 ofreflected power against input power. Comparative plot 3100 depictssimulated results at a 26-km SMF transmission, and includes sub-plots3102 representative of the reflection power (e.g., −35 dBm at 0 dBminput power) measured over different wavelengths of 1528.77 nm, 1548.52nm, and 1567.54 nm, respectively. As can be seen from the exampledepicted in FIG. 31, the reflected power tracks fairly linearly with theinput power, and is substantially agnostic of the various changes to thewavelength.

FIG. 32 is a graphical illustration depicting a comparative plot 3200 ofreflected power against input power. Comparative plot 3200 is similar tocomparative plot 3100, FIG. 31, however, comparative plot 3200 depictssimulated results at a 52-km SMF transmission, and includes sub-plots3202 representative of the reflection power (e.g., again −35 dBm at 0dBm input power) measured over the different wavelengths of 1528.77 nm,1548.52 nm, and 1567.54 nm, respectively. As can be seen from theexample depicted in FIG. 32, the reflected power tracks again fairlylinearly with the input power at this larger transmission distance, andremains substantially agnostic of the various changes to the wavelength.

FIG. 33 is a graphical illustration depicting a comparative plot 3300 ofreflected power against input power. Comparative plot 3300 is similar tocomparative plot 3100, FIG. 31, however, comparative plot 3300 depictssimulated results at a 78-km SMF transmission, and includes sub-plots3302 representative of the reflection power (e.g., again −35 dBm at 0dBm input power) measured over the different wavelengths of 1528.77 nm,1548.52 nm, and 1567.54 nm, respectively. As can be seen from theexample depicted in FIG. 33 as well, the reflected power still tracksfairly linearly with the input power at even larger transmissiondistances, and continues to remain substantially agnostic of the variouschanges to the wavelength.

FIG. 34 is a graphical illustration depicting a comparative plot 3400 ofreflected power against input power. Comparative plot 3400 depictssimulated results at the 1528.77 nm wavelength, and includes sub-plots3402 representative of the −35 dBm reflection power (i.e., at 0 dBminput power) measured over the different transmission distances of26-km, 52-km, and 78-km, respectively. As can be seen from the exampledepicted in FIG. 34, the reflected power tracks fairly linearly with theinput power, and is substantially agnostic of the various changes to thetransmission distances at this wavelength.

FIG. 35 is a graphical illustration depicting a comparative plot 3500 ofreflected power against input power. Comparative plot 3500 depictssimulated results at the 1548.52 nm wavelength, and includes sub-plots3502 representative of the −35 dBm reflection power (i.e., at 0 dBminput power) measured over the different transmission distances of26-km, 52-km, and 78-km, respectively. As can be seen from the exampledepicted in FIG. 35, the reflected power continues to remain fairlylinear with respect to the input power, and is also fairly agnostic ofthe various changes to the transmission distances at this wavelength.However, a slight separation between the respective subplots 3502 cannow be seen at the greater transmission distances.

FIG. 36 is a graphical illustration depicting a comparative plot 3600 ofreflected power against input power. Comparative plot 3600 depictssimulated results at the 1567.54 nm wavelength, and includes sub-plots3602 representative of the −35 dBm reflection power (i.e., at 0 dBminput power) measured over the different transmission distances of26-km, 52-km, and 78-km, respectively. As can be seen from the exampledepicted in FIG. 36, the reflected power it is still substantiallylinear with respect to the input power, and is also somewhat agnostic ofthe various changes to the transmission distances at this higherwavelength. However, at this higher wavelength, the small separationbetween the respective subplots 3502 may nevertheless be seen morereadily as the transmission distance increases.

Full-Duplex Coherent Passive Optical Networks

The embodiments described herein advantageously enable a number ofunique architectures that provide for efficient implementation with acoherent passive optical network (PON). For example, coherent PONarchitectures for implementing the present techniques may includesymmetrical and/or asymmetrical modulation schemes for downstream andupstream communications. In exemplary embodiments of the present systemsand methods, up-conversion and down-conversion may be performed in thedigital domain to mitigate the effects of Rayleigh Backscattering (RB)crosstalk noise (described above) for different reach and splittingratio scenarios.

Conventional PON-based fiber-to-the-home (FTTH) networks are presentlyexpected to deliver more capacity and bandwidth per user by increasingthe bit rate at the OLT and ONU optical transceivers in order to satisfythe continuously growing traffic growth on these networks. However,although the relatively primitive signaling scheme used in theseconventional access networks enables the use of low-cost equipment, theconventional signaling scheme ultimately significantly diminishes thebandwidth that is available to the end-users.

Coherent communication systems offer significantly improvedpower-efficiency and bandwidth-efficiency in comparison with the moreprimitive optical access networks. Coherent communication technology istheoretically able to feasibly transform the conventional accessnetworks and enable ubiquitous new services and applications withuncontended, multi-gigabits-per-user broadband connections.Nevertheless, the more advanced technology of coherent communicationsystems is not readily capable of simply substituting for existingportions of conventional optical access networks, such as in a “plug andplay” manner. Implementation of coherent technology into optical accessnetworks requires significant modifications for the integrationtherewith to become economically viable.

Accordingly, in some exemplary embodiments described herein, in order tominimize system costs, a single laser source may be implemented at thetransmitter side, or hub, to share for both the coherent transmitter andthe coherent receiver at the ONU. In such embodiments, a uniquewavelength may be provided for the downstream and upstreamtransmissions, respectively. In other embodiments, coherent technologymay be uniquely integrated with some conventional technology schemes,such that some overlap between the downstream and upstream transmissionsmay occur. According to the present embodiments, coherent PONs arecapable of realizing full duplex coherent optics in point-to-multipoint(P2MP) configurations, and achieving realistic and efficientbidirectional (BiDi) connections.

FIG. 37 is a schematic illustration of a coherent optics networkarchitecture 3700. Architecture 3700 is similar to architecture 100,FIG. 1, in general operation, and includes a transmitter end 3702, areceiver end 3704, and a transport medium/fiber 3706. Transmitter end3702 may represent a hub, and includes a downstream coherent transceiver3708. In an exemplary embodiment, downstream coherent transceiver 3708includes one or more of a downstream laser 3710, a downstream coherenttransmitter 3712, and a downstream coherent receiver 3714. Downstreamcoherent receiver 3714 is depicted, in this example, as a burst modecoherent receiver. In an embodiment, transmitter end 3702 furtherincludes a three-port downstream optical circulator 3716.

Receiver end 3704 includes a plurality of upstream coherent transceivers3718. Each of upstream coherent transceivers 3718 may represent a nodeor an end user, and includes one or more of an upstream laser 3720, anupstream coherent receiver 3722, and an upstream coherent transmitter3724. In an embodiment, receiver end 3704 further includes a three-portupstream optical circulator 3726 for each coherent transceiver 3718.Each of upstream coherent transceivers 3718 communicates over at leastone short fiber 3728, and are combined onto transport medium 3706 by acombiner 3730.

In an exemplary embodiment, architecture 3700 is configured to implementboth downstream and upstream coherent transmission and reception for aPON configuration. In this example, architecture 3700 is configured totransmit wavelength λ from a burst mode coherent receiver in theupstream direction, and broadcast and select in the downstreamdirection.

FIG. 38 is a schematic illustration of a coherent optics networkarchitecture 3800. Architecture 3800 is generally similar, in overallfunction and several structural elements, to architecture 3700, FIG.3700. In the exemplary embodiment, architecture 3800 thus similarlyincludes a transmitter end 3802, a receiver end 3804, and a transportmedium/fiber 3806. Elements designated by the same label as elements inother drawings may be considered to have similar function and structure.Architecture 3800 differs though, from architecture 3700 in that,whereas architecture 3700 is configured to implement both downstream andupstream coherent transmission and reception

Accordingly, transmitter end 3802 may also represent a hub, and includea downstream coherent transceiver 3808. In the exemplary embodimentdepicted in FIG. 38, downstream coherent transceiver 3808 includes oneor more of a downstream laser 3810, a downstream coherent transmitter3812, and a downstream receiver 3814. In this example, downstreamreceiver 3814 is a burst mode intensity receiver. In an embodiment,transmitter end 3802 further includes a three-port downstream opticalcirculator 3816.

Receiver end 3804 includes a plurality of upstream coherent transceivers3818. Each of upstream coherent transceivers 3818 includes an upstreamcoherent receiver 3820 configured to receive the coherent transmissionfrom downstream coherent transmitter 3812, and an upstream intensitymodulation transmitter 3822 configured to receive and modulate anupstream signal 3824 for transmission to downstream burst mode intensityreceiver 3814. In an embodiment, receiver end 3804 further includes athree-port upstream optical circulator 3826 for each coherenttransceiver 3818. Each of upstream coherent transceivers 3818communicates over at least one short fiber 3828, and are combined ontotransport medium 3806 by a combiner 3830.

In an exemplary embodiment, architecture 3800 is configured to implementan asymmetrical modulation scheme for wavelength λ, using coherenttransmission and reception for downstream communications, andamplitude/intensity modulation and direct detection for upstreamcommunications. In some embodiments, architecture 3800 is configured toimplement, direct detection by external modulation. In otherembodiments, direct detection is implemented by use of a reflectivesemiconductor optical amplifier (RSOA) configured to combineamplification and modulation functionality within a single device.Exemplary ONU structures for enabling such direct detectionimplementations are described further below with respect to FIGS. 39-41.

FIG. 39 is a schematic illustration of an exemplary ONU 3900. In anexemplary embodiment, ONU 3900 is configured to implement externalmodulation and/or an external modulation scheme for upstreamcommunications (e.g., at the receiver end of a communication networkarchitecture). As depicted in FIG. 39, ONU 3900 includes one or more ofan integrated coherent receiver (ICR) 3902, an analog-to-digitalconverter (ADC) 3904, a receiver digital signal processor (DSP) 3906, anoptical coupler 3908, a local oscillator 3910, and a modulator 3912.

In exemplary operation, ONU 3900 is configured to receive a downstreamoptical signal 3914 (e.g., from a downstream transmitter at a hub) atICR 3902, which is then converted by ADC 3904, processed by receiver DSP3906, and then output as reception data 3916. In an exemplaryembodiment, ICR 3902 is also configured to receive, throughcommunication with optical coupler 3908, a local oscillator signal fromlocal oscillator 3910. In further exemplary operation, modulator 3912 isconfigured to receive transmission data 3918, modulate transmission data3918 with the local oscillator signal from local oscillator 3910 (i.e.,also through communication with optical coupler 3908), and output anupstream optical signal 3920.

FIG. 40 is a schematic illustration of an exemplary ONU 4000. In anexemplary embodiment, ONU 4000 is similar to ONU 3900, FIG. 39, in manystructural and functional aspects. ONU 4000 though, differs from ONU3900 in that ONU 4000 is configured to implement RSOA modulation and/oran RSOA modulation scheme for upstream communications.

As depicted in FIG. 40, ONU 4000 similarly includes one or more of anICR 4002, an ADC 4004, a receiver DSP 4006, an optical coupler 4008, anda local oscillator 4010. Different from ONU 3900, instead of a modulator(e.g., modulator 3912, FIG. 39), ONU 4000 implements an RSOA 4012 and anoptical circulator 4014 (a three-port optical circulator, in thisexample).

In exemplary operation, ONU 4000 is similarly configured to such ICR4002 is configured to receive both a downstream optical signal 4016 andthe local oscillator signal from local oscillator 4010. These signalsare then converted by ADC 4004, processed by receiver DSP 4006, andoutput as reception data 4018. In further exemplary operation, ONU 4000may also be configured such that RSOA 4012 is configured to receivetransmission data 4020, and then amplify transmission data 4020 forcombination, at optical circulator 4014, with the local oscillatorsignal from local oscillator 4010 (i.e., through communication withoptical coupler 4008), and output an upstream optical signal 4022.

The exemplary configuration of ONU 4000 may, for example, be ofparticular advantageous use in implementations where a relatively largerpower budget is desired/required (e.g., for longer distancetransmissions). In comparison with an external modulator (e.g., ONU3900), ONU 4000 may provide a lower cost option that reduces therelative LO power requirements.

FIG. 41 is a schematic illustration of an exemplary ONU 4100. In anexemplary embodiment, ONU 4100 is similar to ONU 4000, FIG. 40, in manystructural and functional aspects, but provides an alternativeoperational configuration for implementing RSOA modulation and/or anRSOA modulation scheme for upstream communications. That is, as depictedin FIG. 41, ONU 4100 similarly includes one or more of an ICR 4102, anADC 4104, a receiver DSP 4106, an optical coupler 4108, a localoscillator 4110, an RSOA 4112, and an optical circulator 4114.

In exemplary operation, ONU 4100 is configured to such ICR 4002 isconfigured to receive both a downstream optical signal 4116 (throughoptical coupler 4108) and the local oscillator signal directly fromlocal oscillator 4110. These signals are then converted by ADC 4104,processed by receiver DSP 4106, and output as reception data 4118. Infurther exemplary operation, ONU 4100 is also configured such that RSOA4112 receives transmission data 4120, and then amplifies transmissiondata 4120 for combination, at optical circulator 4114, with downstreamoptical signal 4116 (i.e., through communication with optical coupler4008, in this alternative configuration), and output an upstream opticalsignal 4022.

The exemplary configuration of ONU 4100 may realize similar benefits tothose achieved according to ONU 4000, with respect to longer distancetransmissions relative LO power requirements. ONU 4100 may realize stillfurther advantages with respect to implementations where it is desirableto combine downstream and upstream optical signals, and particularlywith respect to full duplex communications.

The foregoing embodiments illustrate and describe some particularschemes for implementing up/down-conversion in the digital domain tomitigate Rayleigh Backscattering in full duplex coherent opticalsystems. These embodiments are provided though, by way of example, andnot in a limiting sense. That is, the person of ordinary skill in theart will appreciate that the architectures described herein are notlimited to only the coherent signal generation and reception techniquesdescribed above. The present systems and methods may be advantageouslyimplemented where different coherent signal generation and receptiontechniques and architectures are provided. One such alternativeconversion architecture is described below with respect to FIG. 42.

FIG. 42 is a schematic illustration of an exemplary optical conversionarchitecture 4200. Optical conversion architecture 4200 may beadvantageously useful for either or both of digital up-conversion anddigital down-conversion. In an exemplary embodiment, architecture 4200is configured for conversion and complex path mixing (or splitting), andmay be implemented with one or more of the embodiments described herein.

In the exemplary embodiment, architecture 4200 receives input signal4202. Input signal 4202 may represent, for example, a plurality of QAMsymbols. Architecture 4200 further includes one or more of a digitalfilter 4204, a mixer 4206, a summing unit 4208, a digital-to-analogconverter (DAC) 4210, a laser 4212, and a modulator 4214. In anembodiment, summing unit 4208 may be a summing amplifier, and DAC 4210may be configured to convert the I and Q components into separatepathways before the respective components are modulated by modulator4214.

In exemplary operation, at mixer 4206, the filtered QAM symbols aresubject to e^(−2*rr*f) ^(d) ^(*t), in the case where conversionarchitecture 4200 is implemented for the upstream communication signals,or e^(2*rr*f) ^(u) ^(*t), in the case where conversion architecture 4200is implemented for the downstream communication signals. In theexemplary embodiment depicted in FIG. 42, architecture 4200 may thusoperate considering the downstream frequency f_(d) (indicated, forexample, as plot 4216) as being the same as the upstream frequency f_(u)(indicated, for example, as plot 4218), or f=f_(d)=f_(u).

FIG. 43 is a graphical illustration 4300 depicting relative signaldistributions 4302, 4304, 4306, 4308, 4310 for architecture 3700, FIG.37. In an exemplary embodiment, signal distribution 4302 represents aspectral plot seen at a downstream coherent transmitter (e.g.,downstream coherent transmitter 3712), signal distribution 4304represents a spectral plot seen at an upstream coherent transmitter(e.g., upstream coherent transmitter 3724), signal distribution 4306represents a spectral plot seen over a fiber link (e.g., transportmedium 3706), signal distribution 4308 represents a spectral plot seenat an upstream coherent receiver (e.g., upstream coherent receiver3722), and signal distribution 4310 represents a spectral plot seen atan downstream coherent receiver (e.g., downstream coherent receiver3714).

In exemplary operation, signal distribution 4302 represents asubstantially “pure” downstream transmission signal 4312 from thedownstream coherent transmitter, and signal distribution 4304 representsa substantially pure upstream transmission signal 4314 from the upstreamcoherent transmitter. In the exemplary embodiment, both of transmissionsignals 4312, 4314 represent coherent optical signals centered around afrequency f, but where the respective upstream and downstream centerfrequencies are effectively frequency negatives of one another about azero point on the frequency spectrum (e.g., f and “−f,” such as throughoperation of mixer 4206, FIG. 42). Thus, at the fiber link, signaldistribution 4306 depicts a relatively “clean” combination of both puretransmission signals 4312, 4314.

Nevertheless, as indicated by signal distributions 4308, 4310, thespectral distribution recovered at the respective upstream anddownstream receivers is subject to a bleed over effect of the combinedtransmission signals 4312, 4314 on the fiber link. More particularly,although signal distribution 4308 indicates that the downstream coherentreceiver receives downstream transmission signal 4312 substantiallyintact, the downstream coherent receiver also receives a downstreambleed over signal 4316 of upstream transmission signal 4314. That is,downstream bleed over signal 4316 has a frequency distribution thatsubstantially corresponds to a frequency distribution of upstreamtransmission signal 4314, but at a significantly reduced amplitude.

Similarly, as indicated by a signal distribution 4310, the upstreamcoherent receiver receives upstream transmission signal 4314substantially intact, but also an upstream bleed over signal 4318 thatsubstantially corresponds to the frequency distribution of downstreamtransmission signal 4312, but a significantly lower amplitude. Accordingto the exemplary systems and methods described herein, a full duplexcommunication architecture is advantageously able to transmit andreceive the respective upstream and downstream coherent optical signalssimultaneously over the same fiber link, but without substantialinterference to one coherent transmission from the other. By effectivelyseparating the downstream signal from the upstream signal (e.g., byoperation of exemplary up-conversion and down-conversion techniques),the bleed over signal portions may be substantially ignored at therespective receiver.

Exemplary systems and methods of mitigating bleed over effects in fullduplex communication networks are described in greater detail inco-pending U.S. patent application Ser. No. 16/177,428, filed Nov. 1,2018, the disclosure of which is incorporated by reference herein.Additionally, the person of ordinary skill in the art will understandthat the present embodiments are applicable to full duplex coherentcommunications with and without the bleed over effect, and that theembodiments herein are simplified for ease of explanation, and do notnecessarily illustrate all components that may be implemented at thetransmitter portion or the receiver portion, or within a hub or a fibernode.

FIG. 44 is a graphical illustration 4400 depicting relative signaldistributions 4402, 4404, 4406, 4408, 4410 for architecture 3800, FIG.38. Graphical illustration 4400 is therefore similar to graphicalillustration 4300, FIG. 43, except that illustration 4400 depicts a casewhere coherent optical transmission is one-way, namely, in thedownstream direction (e.g., by downstream coherent transmitter 3812).Transmission in the upstream direction is according to intensitymodulation (e.g., by upstream intensity modulation transmitter 3822), inthis example.

In an exemplary embodiment, signal distribution 4402 represents aspectral plot seen at the downstream coherent transmitter, signaldistribution 4404 represents a spectral plot seen at the upstreamintensity modulation transmitter, signal distribution 4406 represents aspectral plot seen over a fiber link (e.g., transport medium 3806),signal distribution 4308 represents a spectral plot seen at an upstreamcoherent receiver (e.g., upstream coherent receiver 3820), and signaldistribution 4310 represents a spectral plot seen at an downstreamreceiver (e.g., downstream burst mode intensity receiver 3814).

In exemplary operation, signal distribution 4402 represents asubstantially pure downstream transmission signal 4412 from thedownstream coherent transmitter, and signal distribution 4404 representsa substantially pure upstream transmission signal 4414 from the upstreamintensity modulation transmitter. In the exemplary embodiment,downstream transmission signal 4412 represents a coherent optical signalcentered around a frequency f, and upstream transmission signal 4414represents an intensity modulated optical signal centered around thezero point on the frequency spectrum, with a bandwidth between thefrequency f and its respective negative. At the fiber link, signaldistribution 4406 depicts a combination of transmission signals 4412,4414, which are simultaneously transmitted in this example.

As indicated by signal distributions 4408, 4410, the recovered spectraldistribution at the respective upstream and downstream receivers in thisembodiment is also subject to a bleed over effect of the combinedtransmission signals 4412, 4414 on the fiber link. That is, in signaldistribution 4408, the downstream coherent receiver receives downstreamtransmission signal 4412 substantially intact, but also receives adownstream bleed over signal 4416 of upstream transmission signal 4414(i.e., substantially the same frequency distribution but loweramplitude). Similarly, in signal distribution 4410, the upstreamintensity modulated receiver is shown to receive upstream transmissionsignal 4414 substantially intact, but also an upstream bleed over signal4418 substantially corresponding to the frequency distribution ofdownstream transmission signal 4412, but at a lower amplitude.

The systems and methods described herein are therefore advantageouslycapable of resolving the deficiencies of conventional coherenttransceiver systems that produce significant crosstalk. As describedwith respect to the embodiments herein, this crosstalk problem issubstantially mitigated or essentially eliminated according to thepresent techniques. According to the innovative embodiments illustratedand described herein, an operator is able to realize significantlyimproved spectral efficiency (e.g., at least double) of existing fibers,whether for single channel or WDM channel operation, and withoutrequiring significant regard to the transmission distance of thefiber(s), or to the particular wavelength(s) transmitted over thechannel(s).

Exemplary embodiments of full duplex coherent optics systems and methodsfor communication networks are described above in detail. The systemsand methods of this disclosure though, are not limited to only thespecific embodiments described herein, but rather, the components and/orsteps of their implementation may be utilized independently andseparately from other components and/or steps described herein.

Although specific features of various embodiments of the disclosure maybe shown in some drawings and not in others, this convention is forconvenience purposes and ease of description only. In accordance withthe principles of the disclosure, a particular feature shown in adrawing may be referenced and/or claimed in combination with features ofthe other drawings.

Some embodiments involve the use of one or more electronic or computingdevices. Such devices typically include a processor or controller, suchas a general purpose central processing unit (CPU), a graphicsprocessing unit (GPU), a microcontroller, a reduced instruction setcomputer (RISC) processor, an application specific integrated circuit(ASIC), a programmable logic circuit (PLC), a field programmable gatearray (FPGA), a digital signal processing (DSP) device, and/or any othercircuit or processor capable of executing the functions describedherein. The processes described herein may be encoded as executableinstructions embodied in a computer readable medium, including, withoutlimitation, a storage device and/or a memory device. Such instructions,when executed by a processor, cause the processor to perform at least aportion of the methods described herein. The above examples areexemplary only, and thus are not intended to limit in any way thedefinition and/or meaning of the term “processor.”

This written description uses examples to disclose the embodiments,including the best mode, and also to enable any person skilled in theart to practice the embodiments, including making and using any devicesor systems and performing any incorporated methods. The patentable scopeof the disclosure is defined by the claims, and may include otherexamples that occur to those skilled in the art. Such other examples areintended to be within the scope of the claims if they have structuralelements that do not differ from the literal language of the claims, orif they include equivalent structural elements with insubstantialdifferences from the literal language of the claims.

What is claimed is:
 1. A coherent optical local transceiver configuredfor communication with a coherent optical remote transceiver over anoptical transport medium in a full duplex communication network, thecoherent optical local transceiver comprising: a local coherenttransmitter configured to transmit a local optical signal, within afirst wavelength frequency band having a first center frequency, overthe optical transport medium to a remote coherent receiver of thecoherent optical remote transceiver; a local coherent receiverconfigured to simultaneously receive a remote optical signal, within asecond wavelength frequency band having a second center frequency, overthe optical transport medium from a remote coherent transmitter of thecoherent optical remote transceiver; a digital filter configured toobtain a filtered digital signal from the received remote opticalsignal; and a conversion unit configured to implement one of digitalup-conversion and digital down-conversion on the filtered digital signalin the digital domain such that the received remote optical signal maybe processed by the local coherent receiver at a conversion frequencydifferent from the second center frequency.
 2. The local transceiver ofclaim 1, wherein the remote optical signal is a burst mode coherentoptical signal.
 3. The local transceiver of claim 1, comprising anoptical network unit.
 4. The local transceiver of claim 3, wherein theoptical network unit comprises one or more of an integrated coherentreceiver, an analog-to-digital converter, a digital signal processor, anoptical coupler, and a local oscillator.
 5. The local transceiver ofclaim 4, wherein the optical network unit further comprises an opticalcirculator and a reflective semiconductor optical amplifier.
 6. Thelocal transceiver of claim 4, wherein the optical network unit furthercomprises a modulator.
 7. The local transceiver of claim 5, wherein theoptical circulator is configured to combine the downstream opticalsignal with an output from the reflective semiconductor opticalamplifier.
 8. The local transceiver of claim 5, wherein the opticalcirculator is configured to combine an output from the local oscillatorwith an output from the reflective semiconductor optical amplifier. 9.The local transceiver of claim 6, wherein the optical network unit isconfigured to implement direct detection using external modulation. 10.The local transceiver of claim 5, wherein the optical network unit isconfigured to implement direct detection using external reflectivesemiconductor optical amplifier modulation.
 11. The local transceiver ofclaim 1, wherein the conversion unit comprises a filter, a mixer, asumming unit, a digital-to-analog converter, a laser, and a modulator.12. The local transceiver of claim 11, wherein the digital-to-analogconverter is configured to convert an I component and a Q component ofan input signal.
 13. The local transceiver of claim 12, wherein theinput signal comprises a plurality of QAM symbols.
 14. The localtransceiver of claim 13, wherein the mixer is configured to mix theplurality of QAM symbols with a polynomial based on the first and secondcenter frequencies, and wherein the polynomial includes a positive ornegative exponent according to whether the conversion unit is applied tothe upstream optical signal or the downstream optical signal.
 15. Alocal transceiver configured for communication with a remote transceiverover an optical transport medium in a full duplex communication network,the local transceiver comprising: a local coherent transmitterconfigured to transmit a local coherent optical signal, within a firstwavelength frequency band having a first center frequency, over theoptical transport medium to a remote coherent receiver of the remotetransceiver; a local burst mode intensity receiver configured tosimultaneously receive a remote intensity-modulated optical signal,within a second wavelength frequency band having a second centerfrequency within the first wavelength frequency band, over the opticaltransport medium from a remote intensity modulation transmitter of theremote transceiver; and a conversion unit configured to digitallyup-convert or digitally down-convert the received remoteintensity-modulated optical signal to remove bleed-over from thereceived remote intensity-modulated optical signal into the transmittedlocal coherent optical signal.